This invention relates to protective coatings for components exposed to high temperatures, such as the hostile thermal environment of a gas turbine engine. More particularly, this invention is directed to a method of stabilizing the microstructure of a thermal barrier coating (TBC) with carbide and/or nitride-based precipitates in order to inhibit degradation of the thermal insulating properties of the TBC during high temperature excursions.
Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components within the hot gas path of the engine must correspondingly increase. Significant advances in high temperature capabilities have been achieved through the formulation of nickel and cobalt-base superalloys. Nonetheless, when used to form components of the turbine, combustor and augmentor sections of a gas turbine engine, such alloys alone are often susceptible to damage by oxidation and hot corrosion attack and may not retain adequate mechanical properties. For this reason, these components are often protected by a thermal barrier coating (TBC) system. TBC systems typically include an environmentally-protective bond coat and a thermal-insulating ceramic topcoat, typically referred to as the TBC. Bond coat materials widely used in TBC systems include oxidation-resistant overlay coatings such as MCrAlX (where M is iron, cobalt and/or nickel, and X is yttrium or another rare earth element), and oxidation-resistant diffusion coatings such as diffusion aluminides that contain aluminum intermetallics.
Ceramic materials and particularly binary yttria-stabilized zirconia (YSZ) are widely used as TBC materials because of their high temperature capability, low thermal conductivity, and relative ease of deposition by plasma spraying, flame spraying and physical vapor deposition (PVD) techniques. TBC""s employed in the highest temperature regions of gas turbine engines are often deposited by electron beam physical vapor deposition (EBPVD), which yields a columnar, strain-tolerant grain structure that is able to expand and contract without causing damaging stresses that lead to spallation. Similar columnar microstructures can be produced using other atomic and molecular vapor processes, such as sputtering (e.g., high and low pressure, standard or collimated plume), ion plasma deposition, and all forms of melting and evaporation deposition processes (e.g., cathodic arc, laser melting, etc.).
In order for a columnar TBC to remain effective throughout the planned life cycle of the component it protects, it is important that the TBC maintains a low thermal conductivity throughout the life of the component. However, the thermal conductivities of TBC materials such as YSZ are known to increase over time when subjected to the operating environment of a gas turbine engine. As a result, TBC""s for gas turbine engine components are often deposited to a greater thickness than would otherwise be necessary. Alternatively, internally cooled components such as blades and nozzles must be designed to have higher cooling flow. Both of these solutions are undesirable for reasons relating to cost, component life and engine efficiency.
In view of the above, it can be appreciated that further improvements in TBC technology are desirable, particularly as TBC""s are employed to thermally insulate components intended for more demanding engine designs.
The present invention generally provides a thermal barrier coating (TBC) and method for forming the coating on a component intended for use in a hostile environment, such as the superalloy turbine, combustor and augmentor components of a gas turbine engine. The coating and method are particularly directed to inhibiting grain growth, sintering and pore coarsening or coalescence in the TBC during high temperature excursions. Improvements obtained by this invention are particularly evident with TBC having a columnar grain structure, such as those deposited by EBPVD and other PVD techniques, though the invention is also applicable to TBC deposited by such methods as plasma spraying.
In TBC""s having a columnar grain structure, heat transfer through the TBC is primarily by conduction through the individual columnar grains. According to the invention, resistance to heat transfer through the TBC is believed to be enhanced by microstructural defects within the grains created by composition-induced defect reactions and process-induced porosity. As used herein, composition-induced defect reactions include vacancies that result from the need in ionic solids to maintain charge neutrality, as is the case in YSZ where substitution of zirconia (ZrO2) with yttria (Y2O3) in the lattice yields a vacancy. These lattice defects cannot be controlled through microstructural manipulation, as the atomic defects are based solely on thermodynamics and are not process-dependent. Therefore, compositional changes (substitutional changes that affect defect reactions) are the only way to affect the concentration of this type of defect. Process-induced porosity includes pore formation that occurs as a component being coated is rotated relative to the deposition source. A primary example is the xe2x80x9csunrise-sunsetxe2x80x9d vapor-surface mechanisms that occur during rotation of a component during deposition of TBC from a vapor cloud, such as by PVD, the result of which is a textured growth of the deposit in which pores are formed between columns, within the columns, and between secondary growth arms contained within the columns. In order for a columnar TBC to maintain a low thermal conductivity throughout the life of the component it protects, process-induced porosity must be preserved to stabilize the associated microstructural defects. However, microstructures of TBC materials such as YSZ have been found to sinter, coarsen and undergo pore redistribution (as used herein, when smaller pores coalesce or coarsen to form larger pores) during high temperature exposures, such as temperatures in excess of 1000xc2x0 C. found within the hot gas path of a gas turbine engine. As a result, increases in thermal conductivity noted for TBC materials on gas turbine engine components can be attributed to sintering, grain coarsening and pore redistribution.
As a solution, this invention inhibits TBC grain growth and pore redistribution with limited amounts of extremely fine carbide-based and/or nitride-based precipitates formed at the defects and pores of the TBC microstructure, as well as on the grain boundaries of the TBC. Preferred carbide and nitride-based precipitates include metal carbides and nitrides, oxycarbides and carbonitrides. The precipitates are for the purpose of pinning the TBC grain boundaries to inhibit sintering, grain coarsening and pore redistribution within the TBC microstructure during high temperature excursions, with the effect that the microstructure, and consequently the thermal conductivity of the TBC, is stabilized. To be effective, the precipitates must be smaller than the pores intended to be stabilized, which are typically on the order of about fifty nanometers up to about one or two micrometers. In addition, the volume fraction of the precipitates is preferably on the order of about three to twenty-five volume percent. The lower volume limit is the minimum amount of precipitates that is believed would have any significant effect, while the upper limit corresponds to the approximate volume attributable to pores within the TBC microstructure, e.g., formed by process-induced porosity.
A suitable method for carbiding/nitriding the TBC microstructure is to deposit the TBC using a physical vapor deposition technique in an atmosphere that contains carbon and/or nitride gases or compounds thereof, depending on whether precipitates of carbides, nitrides or both are desired. In this manner, the TBC and the desired precipitates are simultaneously formed. An additional and optional step is to heat treat the component in the presence of a gas containing carbon and/or nitrogen gases or compounds thereof to form an additional amount of precipitates in the TBC.
By sufficiently stabilizing the TBC microstructure and pinning grain boundaries with carbide and/or nitride precipitates, the component can be subsequently heated to temperatures in excess of 1200xc2x0 C. without causing sintering, grain coarsening and pore redistribution of the microstructure. As a result, components can be designed for thinner TBC and/or, where applicable, lower cooling air flow rates, which reduces processing and material costs and promotes component life and engine efficiency.
Other objects and advantages of this invention will be better appreciated from the following detailed description.